Reducing the prevalence of obesity in the population is one of the Healthy People 2010 goals, and active vigilance is required in all age groups in preventing and reversing overweight and obesity. On a population basis, the prevalence of overweight is associated with a myriad of influences, including social, behavioral, cultural and environmental factors as well as genetic and physiological factors. For a majority of overweight individuals, restoring a balance between energy intake and expenditure is difficult, and therefore there is an increasing emphasis on preventing obesity on a population level. The longer-term goal of this research is to prevent further increase in rates of obesity in the population. This project has the potential to influence the worksite environment in ways informed by this Center's other projects. In the adult population of working age, a majority of the day is spent in the worksite, suggesting that interventions at the worksite level may offer the opportunity for success in this age group. We propose to develop and test a comprehensive intervention with simple messages that will integrate changes in dietary intake with changes in energy expenditure, while simultaneously modifying structural and environmental factors to promote social support and opportunities for behavioral change. We will recruit and randomize 28 worksites, from the 98144 zip code area, to a two year intervention in which we will: build a physical activity intervention combining increased daily physical activity and regular, structured exercise;build a dietary intervention that will promote lower calorie intake;increase worksite access to both healthy foods and physical activity. Our primary aim is to evaluate the effectiveness of the intervention in reducing or maintaining body mass index in a randomized controlled trial of worksites. We will compare changes in body mass index in intervention versus control worksites using two cross-sectional surveys at baseline and followup. The impact of the intervention on biomarkers related to nutritional intake, obesity, inflammation, insulin resistance and adipokines will be estimated in a subset of employees. Our team has considerable experience with interventions at the worksite level and substantial expertise in obesity prevention. We anticipate that this project will yield important contributions to the implementation of obesity prevention and will be informed by study results obtained in Projects 1-4.